1. Field of the Invention
The invention relates to a vehicle attitude control system that includes a control unit that controls front wheels and rear wheels.
2. Discussion of Background
A conventional vehicle attitude control system calculates a target reference yaw rate on the basis of information, such as a vehicle speed and a steering angle, and controls front wheels and rear wheels on the basis of a deviation of a yaw rate detected by a yaw rate sensor from the reference yaw rate.
Japanese Patent Application Publication No. 2008-94214 (JP 2008-94214 A) describes an example of a vehicle attitude control system. The vehicle attitude control system includes yaw moment distribution determining means for determining an operation amount of a steered angle control mechanism and an operation amount of a torque control mechanism such that a vehicle makes a movement based on the reference yaw rate. The yaw moment distribution determining means determines a yaw moment distribution between the steered angle control mechanism and the torque control mechanism on the basis of, for example, the degree of allowance of a tire friction circle of each of front and rear wheels and a frequency component of a target value of yaw moment.
Generally, the reference yaw rate is set on the basis of a vehicle model in which a tire characteristic is within a linear region, that is, a linear vehicle model, and an output from a yaw rate sensor in a state where the vehicle travels while tires are gripping a road surface. However, when there is no deviation of the output from the yaw rate sensor from the reference yaw rate, that is, when the tires are gripping a road surface, the vehicle attitude control system does not execute vehicle attitude control based on the reference yaw rate. Therefore, when the tires are gripping a road surface, it is not possible to appropriately control the travel direction of the vehicle.
In addition, in a vehicle attitude control system that has a computing equation of a reference yaw rate prescribed on the premise of predetermined vehicle specifications, when vehicle specifications (for example, vehicle weight and suspension characteristics) corresponding to the number of occupants, a superimposed load, and the like, have changed from the predetermined vehicle specifications, a reference yaw rate corresponding to the vehicle specifications is not calculated by the computing equation of the prescribed reference yaw rate. Therefore, it is not possible to appropriately control the travel direction of the vehicle.